Studies of the discriminative stimulus attributes of drugs will be continued in a series of experiments on rats, using three prototype sedative drugs (pentobarbital, chlordiazepoxide, ethyl alcohol). Groups of animals will be trained to discriminate between two of these drugs and compared with groups trained to discriminate one drug from the saline control condition. Learning of the discriminative response will be accelerated by a combination of techniques to be tested and used. These techniques include initial high doses of the drugs, long duration of sessions or large number of successive trials in the same training session, several successive sessions under the same drug condition before a series of sessions under the other condition, minimzing the effects of distracting responses by body confinement, with alternative directions of head turn as the discriminative response, and maximizing the suddenness of onset of drug effect by training at short intervals after drug administration. Discrimination on the basis of the qualitative drug stimulus rather than a fixed quantitative level will be trained by equal frequencies of two doses in a series of training sessions. In some experiments, the minimum detectable dose will be measured by the ED50 and will be determined by a procedure of progressively decreasing the two doses. In most experiments, tests under novel drug conditions will establish relative similarity of other doses, time intervals, and drugs to the drug conditions the animals have been trained to discriminate.